Rapid identification of Staphylococci isolated in clinical microbiology laboratories by matrix-assisted laser desorption ionization-time of flight mass spectrometry

Abstract

Matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF-MS) of intact bacteria yields a reproducible spectrum depending upon growth conditions, strain, or species. Using whole viable bacteria we describe here the application of MALDI-TOF-MS to the identification of coagulase-negative staphylococci (CoNS). Our aim was, once a bacterium has been recognized as Micrococcaceae, to identify peaks in the spectrum that can be used to identify the species or subspecies. MALDI-TOF-MS was performed using bacteria obtained from one isolated colony. One reference strain for each of the 23 clinically relevant species or subspecies of Micrococcaceae was selected. For each reference strain, the MALDI-TOF-MS profile of 10 colonies obtained from 10 different passages was analyzed. For each strain, only peaks that were conserved in the spectra of all 10 isolated colonies and with a relative intensity above 0.1 were retained, thus leading to a set of 3 to 14 selected peaks per strain. The MALDI-TOF-MS profile of 196 tested strains was then compared with that of the set of selected peaks of each of the 23 reference strains. In all cases the best hit was with the set of peaks of the reference strain belonging to the same species as that of the tested strain, thus demonstrating that the 23 sets of selected peaks can be used as a database for the rapid species identification of CoNS. Similar results were obtained using four different growth conditions. Extending this strategy to other groups of relevant pathogenic bacteria will allow rapid bacterial identification.

FIG. 1.

MALDI-TOF-MS profiles of 10 isolates of the same strain of S. hominis subsp. hominis CIP 102642. *, peaks constantly obtained.

FIG. 2.

Dendrograms of species of Micrococcaceae. The dendrograms were engineered using databases 1 (spectra obtained from bacteria grown on Mueller-Hinton agar for 24 h) and 2 (spectra obtained from bacteria grown on Mueller-Hinton agar for 48 h). Similar results (not shown) were observed with databases 3 and 4.